1. Field of the Invention
This invention generally relates to calibration of analytical chemistry devices, and more particularly relates to a device for calibrating sensors for measuring gas concentrations and pH of a fluid.
2. Description of Related Art
In modern medicine, measurement of acidity (pH), and oxygen and carbon dioxide levels in the blood has become an important factor in the determination of the respiratory status of a patient. Although electrodes have been developed which are capable of measuring these blood factors in fluids, such electrodes are of limited use in measurement of in vivo blood pH levels. Optical sensors called "optodes" have been developed for taking intravascular measurements of acidity and other blood analytes such as oxygen and carbon dioxide. Such optical sensors typically include a fluorescent indicator dye placed over the tip of an optical fiber and covered by a membrane which is permeable to the chemical of interest.
It is frequently desireable to keep such sensors wet prior to use in an aqueous, tonometered buffer solution which is isotonically adjusted to match the ionic strength of the fluid of interest, such as blood. Such sensors must also be sterilized, such as in an autoclave, before they are used intravenously. The autoclaving process can cause pressure buildup in the buffer solutions, placing unusual stresses on the fluid container in which the sensor is sterilized to cause leakage of the container. It is also desirable to calibrate such sensors before use, and frequently several times daily, using tonometered sample liquids with known levels of the analyte of interest. One method of preparing an appropriate tonometered buffer solution involves bubbling a prepared gas mixture, such as of CO.sub.2, O.sub.2, and N.sub.2 through the solution until equilibration of the gas mixture in the solution occurs. However, it has been found that drying of the membrane of the chemical sensor can occur where gas bubbles come in contact with the chemical sensor, affecting the performance of the sensor. It is also useful to provide a bio-filter in the gas bubbling apparatus to filter the gas mixture before it enters the solution, but it has been found that such filters can become clogged if exposed to the buffer solution for extended storage periods.
Accordingly, there remains a need for an apparatus that will allow storage of the sensor in an appropriate fluid to protect the sensor from drying out, that will provide a way of isolating the bio-filter from becoming clogged during a period of storage of the sensor in the fluid, and that will provide for good sealing of the sensor and fluid in the apparatus for the internal pressure which builds up in the fluid during the autoclaving process.